1. Field of the Invention:
The present invention relates to a photodiode including an anti-reflection film provided on a surface of a light receiving area and a light shielding layer provided in the vicinity of the light receiving area, and a light receiving device with a built-in circuit including such a photodiode.
2. Description of the Related Art:
Conventionally, a photodiode for performing photoelectric conversion has been widely used as, for example, a signal detection device of an optical pickup, or a light receiving device on a receiving side of an optical space transmission system or an optical fiber link. Along with recent improvement in the operation speed of optical pickups of DVD apparatuses and the like, and also along with recent improvement in data transmission speed, there is a demand for a higher performance photodiode having a higher response speed and for a light receiving device with a built -in circuit including such a photodiode. Especially, there is a demand for a photodiode having a higher level of photosensitivity and reduced high frequency noise as well as a higher response speed.
In order to have an improved level of photosensitivity, a photodiode needs to have an antireflection film as a top layer of a light receiving area thereof for reducing the reflectance of light incident on the photodiode and thus preventing the incident light from being lost by reflection at the surface of the light receiving area. Such an anti-reflection film is formed of a lamination structure of a thin silicon oxide layer and a thin silicon nitride layer. The lamination structure is provided as a top layer of the light receiving area of a photodiode, and is set to have an appropriate thickness in accordance with the wavelength of the incident light so as to optimize the reflectance of the incident light. For example, Japanese Laid-Open Publication No. 10-84102 discloses an anti-reflection film formed of a silicon oxide layer having a thickness of 10 nm and a silicon nitride layer having a thickness of 50 nm in order to optimize light, emitted by a semiconductor laser device, having a wavelength commonly used for a CD or DVD-ROM apparatus (e.g., 780 nm).
In the field of optical discs, apparatuses using laser light having a shorter wavelength for reading and writing data are now developed. As the wavelength of the laser light is shorter, the diameter of the optical beam is smaller and so the recording density of optical discs can be improved. Such reduction in the wavelength of the laser light requires the silicon oxide layer and the silicon nitride layer included in the anti-reflection film to be even thinner, for the purpose of reducing the optical loss caused by the reflection at the surface of the photodiode and thus improving the photosensitivity of the photodiode.
In order to have an improved response speed and reduced high frequency noise, a photodiode needs to have a reduced junction capacitance at a P-N junction thereof. This is preferably realized by increasing the resistance of a semiconductor layer included in the light receiving area. However, this technique increases the inner series resistance of the photodiode and therefore does not sufficiently improve the response speed. In an attempt to solve this problem, Japanese Laid-Open Publication No. 8-260043 discloses a photodiode device 1000 having a structure shown in FIG. 17.
The photodiode device 1000 includes a P-type silicon layer 21 and an N−-type epitaxial layer 24 laminated on the P-type silicon layer 21. At an interface between the P-type silicon layer 21 and the N−-type epitaxial layer 24, P+-type buried diffused layers 22 are provided with a prescribed distance therebetween. In the N−-type epitaxial layer 24, a P+-type isolating diffused layer 22a is provided on each P+-type buried diffused layer 22. The P+-type buried diffused layers 22 and the P+-type isolating diffused layers 22a divide the N−-type epitaxial layer 24 into a plurality of regions. A photodiode device 1000 detects incident light by a P-N junction at an interface between each region of the N−-type epitaxial layer 24 and the P-type silicon substrate 21. Thus, a plurality of photodiodes are formed including the interface between the P-type silicon substrate 21 and the N−-type epitaxial layer 24.
Reference numeral 29 represents a light receiving area of the photodiode device 1000. The light receiving area 29 has the following structure. At the interface between the P-type silicon layer 21 and the N−-type epitaxial layer 24, N+-type buried diffused layers 23 are provided in the vicinity of, and so as to interpose, the P+-type buried diffused layer 22 and the P+-type isolating diffused layer 22a, which are provided so as to divide the light receiving area 29. A P+-type diffused layer 26 is provided above, and so as to overlap the N+-type buried diffused layers 23. The P+-type diffused layer 26 includes a part of the P+-type isolating diffused layer 22a. 
The N−-type epitaxial layer 24, the P+-type isolating diffused layer 22a and the P+-type diffused layer 26 is covered with a continuous anti-reflection film 25. The anti-reflection film 25 includes a silicon oxide layer 25a and a silicon nitride layer 25b laminated on the silicon oxide layer 25a. A conductive light shielding layer 27 is provided on portions of the anti-reflection film 25 which are in the vicinity of the light receiving area 29. An insulating passivation layer 28 is provided so as to cover the entirety of the conductive light shielding layer 27.
As described above, the N+-type buried diffused layers 23, which have a low resistance, are provided only in the vicinity of the P+-type buried diffused layer 22 and the P+-type isolating diffused layer 22a, which divide the light receiving area 29. Due to such a structure, the photodiodes have a reduced inner series resistance and a reduced junction capacitance.
The photodiode device 1000 having the above-described structure is first produced as a wafer, and then the wafer is divided into a plurality of chips by a dicing step. During the dicing step, a large amount of electrostatic charges are generated on the surface of the wafer; more specifically, on the light receiving area 29, and on the silicon nitride layer 25b and the insulating passivation layer 28 in an area in the vicinity of the light receiving area 29. Such electrostatic charges may undesirably cause electrostatic destruction of the photodiodes formed below the surface of the wafer. Such a large amount of electrostatic charges are generated by friction between the wafer and pure water sprayed to the wafer during the dicing step. One technique adopted in order to suppress the generation of the electrostatic charges is to mix carbon dioxide gas with the pure water to be sprayed to the wafer so as to increase the conductivity.
Even this technique cannot sufficiently suppress the generation of the electrostatic charges on the surface of the wafer, since a change in the flow rate of the pure water and a change in the flow rate of the carbon dioxide gas change the specific resistance of the pure water. Mixing a great amount of carbon dioxide gas can reduce the specific resistance of the pure water and thus suppress the generation of electrostatic charges, but this technique is not practical because reduction in the specific resistance of the pure water shortens the life of the blade used for dicing the wafer.
The electrostatic charges generated on the silicon nitride layer 25b in the vicinity of the light receiving area 29 generate electric charges at an interface state between the silicon oxide layer 25a and the silicon nitride layer 25b both having a small thickness. The electric charges invert the conductivity type of the semiconductor layer below the silicon oxide layer 25a and the silicon nitride layer 25b. 
For example, the photodiode device 1000 shown in FIG. 17 acts as follows. When positive electrostatic charges are generated on the silicon nitride layer 25b in the vicinity of the light receiving area 29, negative electric charges are accumulated at the interface between the silicon oxide layer 25a and the silicon nitride layer 25b. As a result, a surface area of the N−-type epitaxial layer 24 is inverted to be a P-type semiconductor layer. The resultant P-type semiconductor layer is electrically connected to the P+-type isolating diffused layer 22a and the like, which increases the junction capacitance of the photodiodes.
As described above, optical pickups of recent optical disc apparatuses use laser light having a shorter wavelength. In order to reduce the reflection at the surface of a photodiode, the anti-reflection film of the photodiode has a reduced thickness. When the thickness of the silicon oxide layer included in the anti-reflection film is reduced, the ratio of the electric potential difference (generated by the electrostatic charges) which are applied to the semiconductor layer below the anti-reflection film is increased. Therefore, when the thickness of the anti-reflection film is reduced, the conductivity of the semiconductor layer is more easily inverted. As a result, the junction capacitance of the photodiode may be undesirably further increased by the generation of the electrostatic charges during the dicing step.
An increase in the junction capacitance of the photodiode decreases the response speed and increases the high frequency noise of the photodiode. Thus, the characteristics of the photodiode may be significantly deteriorated.
It is impossible to completely eliminate the generation of the electrostatic charges at the surface of the wafer during the dicing step. It is also impossible to remove the electric charges introduced by the electrostatic charges to the interface between the silicon oxide layer 25a and the silicon nitride layer 25b from the interface unless the interface is irradiated with high energy ultraviolet rays or the like, since such electric charges have a stable potential.
In order to prevent such an increase in the junction capacitance, Japanese Laid-Open Publication No. 11-40790 discloses a photodiode device 2000 shown in FIG. 18. Identical elements as those described with reference to FIG. 17 bear identical reference numerals therewith and detailed descriptions thereof are omitted.
The photodiode device 2000 includes a conductive light shielding layer 27 provided in the vicinity of a light receiving area 29, and an insulating passivation layer 28a provided on the light shielding layer 27 so as to partially cover the light shielding layer 27. More specifically, as shown in FIG. 18, an end 28b of the insulating passivation layer 28a is farther from the light receiving area 29 than an end 27b of the conductive light shielding layer 27. As a result, the end 27b of the conductive light shielding layer 27 is exposed. The photodiode device 2000 has an identical structure to that of the photodiode device 1000 shown in FIG. 17 on the other points.
In the photodiode device 2000 in which the end 27b of the conductive light shielding layer 27 is exposed, the large amount of electrostatic charges (positive charges) generated on the silicon nitride layer 25b can be removed through the end 27b of the conductive light shielding layer 27. Therefore, negative charges are not accumulated at the interface between the silicon oxide layer 25a and the silicon nitride layer 25b of the anti-reflection film 25.
The photodiode device 2000 have the following problems.
The conductive light shielding layer 27 is usually formed of AlSi. Al (aluminum) contained in AlSi has a strong tendency to be ionized. Therefore, when moisture containing impurities adheres to a surface of AlSi layer, electrons are exchanged at a portion of the surface having the moisture, thus corroding AlSi. The conductive light shielding layer 27 is also used as a signal line. Therefore, when the conductive light shielding layer 27 is corroded, the photodiodes suffer from the problems that (i) the light shielding characteristic is deteriorated and (ii) the signal line is ruptured and as a result, the electric signal obtained by the photoelectric conversion of the incident light cannot be detected.
A light receiving device with a built-in circuit including a photodiode includes a polysilicon electrode at a relatively shallow position in order to improve the operating speed of the transistor. When the AlSi layer used for the signal line contacts a polysilicon layer used for the electrode, silicon is deposited at the interface between the AlSi layer and the polysilicon layer. This causes a contact resistance. In order to prevent this, the photodiode device 2000 includes a titanium-tungsten alloy (TiW) layer (not shown) as a barrier metal layer for preventing the deposition of silicon, directly below the conductive light shielding layer 27 formed of AlSi.
However, in the structure shown in FIG. 18 in which the end 27b of the conductive light shielding layer 27 is exposed, the interface between the AlSi layer and the TiW layer is exposed to the outside air. When, for example, pure water is sprayed to the wafer in the dicing step, AlSi may be undesirably dissolved and corroded by a cell reaction. When the conductive light shielding layer 27 formed of AlSi is corroded, the light shielding characteristic of the conductive light shielding layer 27 in the light receiving area 29 is deteriorated, and the effect of preventing the negative charges from accumulating at the interface between the silicon oxide layer 25a and the silicon nitride layer 25b is reduced. When the light shielding characteristic provided by the conductive light shielding layer 27 is deteriorated, light may undesirably be incident on the photodiodes through areas other than the light receiving area 29 and then converted into an electric signal. This increases a noise component, resulting in deterioration of various characteristics of the photodiodes.
The structure shown in FIG. 17 in which the conductive light shielding layer 27 is covered with the insulating passivation layer 28 is less susceptible to corrosion but is more susceptible to electrostatic charges. The yield of photodiodes having the structure shown in FIG. 17 is lower than that of the photodiodes having the structure shown in FIG. 18.